Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
Ideally, the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle.
The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a system pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the system pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known in the art to modify the angular distribution of the projection light in the mask plane so as to achieve the desired illumination setting. For achieving maximum flexibility in producing different angular distribution in the mask plane, it has been proposed to use mirror arrays that illuminate the pupil surface.
In EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) including more than 1000 mirrors. Each of the mirrors can be tilted about two orthogonal tilt axes. Thus radiation incident on such a mirror device can be reflected into almost any desired direction of a hemisphere. A condenser lens arranged between the mirror array and the pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This known illumination system makes it possible to illuminate the pupil surface with a plurality of spots, wherein each spot is associated with one particular microscopic mirror and is freely movable across the pupil surface by tilting this mirror.
Similar illumination systems are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2. Similar arrays of tiltable mirrors have also been proposed for EUV illumination systems.
In such arrays the orientation of the individual mirrors typically has to be controlled with great accuracy and at high speed. To this end it has been proposed to use a closed-loop control. Such a control scheme involves that the orientation of each mirror be monitored with a high repetition frequency.
International application WO 2008/095695 A1 discloses a measurement device which makes it possible to measure the orientation of each individual mirror. To this end an illumination unit is provided which produces for each mirror an individual measuring light beam. A detector unit measures the angle of the light beams after they have been reflected from the mirrors. If the direction of the light beams impinging on the mirrors is known, the orientation of the mirrors can be determined by an evaluation unit on the basis of the measured directions of the reflected light beams. By using time or frequency multiplexing it is possible to distinguish the measuring light beams so that the orientation of the mirrors can be determined sequentially or even in one go.
A known illumination unit uses an array of laser diodes, for example vertical cavity surface emitting lasers (VCSEL), as light sources producing the measuring light beams that are directed on the mirrors. For each laser diode an imaging lens is provided that is arranged in front of the laser diode and images the light exit facet of the diode on one of the mirrors. The imaging lenses preferably form a microlens array having the same pitch as the laser diodes.
However, it has turned out that under the most preferred dimensions of such measurement devices the measurement accuracy is often not satisfactory.
This can also be encountered in similar measurements within the field of microlithographic projection exposure apparatus. For example, in the projection objectives of such apparatus there are sometimes lenses or mirrors having an optical surface which can be deformed in order to correct aberrations. Deformations of the optical surface may be accomplished, for example, with the help of actuators exerting mechanical forces, or by directing radiation on certain areas on the optical surface. For controlling the deformation process the shape of the optical surface can be measured using a measurement device which is configured to measure a parameter related to the optical surface at a plurality of areas. Since it does not really matter whether the optical surface is continuous or discontinuous (as it is the case with a mirror array), the measurement accuracy achievable with such measurement devices is sometimes not satisfactory.